1. Field of Invention
This invention relates to an improved optical gas sensor with temperature compensable performance for the detection and determination of gas concentration by means of absorption spectroscopy, using non-dispersive radiation. More particularly, the present invention relates to an optopair for use in optical gas sensors and/or optical absorption gas analyzers, the optopair comprising an LED source and a corresponding Photodetector capable of reliable and consistent qualitative and quantitative analysis of gases at different temperatures, notwithstanding the optopair's sensitivity to temperature changes.
2. Prior Art
Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of its frequency or wavelength, due to its interaction with a sample to be analyzed (analyte). The analyte absorbs some of the radiation, as it comes in contact with it. The amount of the radiation absorbed by the analyte varies as a function of the frequency of the radiation, and the concentration of the analyte (Beer-Lambert Absorption Law)1, and this variation determines the absorption spectrum of the sample analyte. Thus, absorption spectroscopy is employed as an analytical tool to determine the presence of a particular analyte and, in many cases, to quantify the amount of the analyte present. 1 Soinikova, G. Y., Low Voltage CO2-Gas Sensor based on III-V Mir-IR Immersion Lens Diode Optopairs: Where are we and How Far we Can Go? IEEE SENSORS JOURNAL, 2009
Absorption spectroscopy, as an analytical tool, works by directing a generated beam of radiation at the analyte and detecting the intensity of the radiation that passes through it. The analysis of the transmitted radiation can be used to calculate the absorption and from the absorption the concentration of the analyte. The source, sample arrangement, and detection technique vary significantly depending on the frequency range and the purpose of the experiment.
Absorption spectroscopy methods utilize any type of radiation. Provided however, that the type of radiation utilized depends on the atomic structure and nature of chemical bonds of the analyte. Many gases are highly absorbent in the infrared spectral region. Absorption spectroscopy methods using infra-red radiation have long been recognized as sensitive, stable, and reliable methods for the detection and determination of the concentration of gases, in among other things, atmospheric air. If the measured parameter is the intensity of absorbed radiation at a fixed frequency or within a fixed frequency range, the spectroscopic method is called “non-dispersive.” Such non-dispersive infra-red absorption measurement methods are based on the gases' molecular properties, which enable them to interact with and absorb infrared radiation within a certain spectral range.
When the gases are placed in the path of the infra-red radiation and the radiation's spectrum corresponds to the absorption spectrum of the gases, the gases will absorb such radiation.2 Further, the gases will absorb the most when the wavelengths of maximum radiation intensity correspond to, coincide with, and match the wavelengths of maximum gas radiation absorption. More specifically, in a typical infra-red atomic spectroscopy method, the concentration of the gas of interest in a sample is determined once the absorption of the infra-red radiation by the gas has been detected and measured. As a result, non-dispersive infra-red absorption spectroscopy methods are widely used to detect many different gases, including carbon dioxide, carbon monoxide, methane, ethane, hydrogen sulfide and so on. 2 Sotnikova, G. Y., Low Voltage CO2-Gas Sensor Based on III-V Mid-IR Immersion Lens Diode Optopairs: Where are we and how far can We go? IEEE Sensors Journal, 2009.
Infra-red absorption spectroscopy instrumentation comes in more than one configuration. A typical “one channel” infra-red instrument (“sensor”) comprises a source of radiation (usually infrared), such as an incandescent lamp or another electrically heated element that serves as a blackbody emitter, e.g., a silicon carbide rod or nichrome filament; a narrow bandpass filter arranged to ensure that only radiation intensity absorbed by the gas of interest is measured; a gas chamber for containing a sample including the target gas of interest; and a photodetector for detecting radiation transmitted by the sample and transforming the energy of the detected radiation into an electrical signal whose magnitude corresponds to the intensity of the detected radiation.
“Two channel” infra-red sensors have a signal channel and a reference channel. The signal channel operates in exactly the same way as the “one” channel device described above, with the transmission band of the band pass filter adjusted to the absorption wavelength(s) of the gas of interest. The reference channel usually works in another wavelength band, at which the target gas species does not absorb. This provides a base line for the signal channel. The differential signal between the signal and reference channels, normalized on reference channel intensity, gives an absorption signal which is stable with respect to any intensity drift resulting from the radiation source (or detector).
Another type of a “two channel” infra-red sensor comprises two photodetectors and includes two separate gas cells into which the emission from the radiation source is split along paths of equal lengths. One cell is filled with nonabsorptive (inert) gas to provide a reference channel, and the other with the sample gas (including the gas of interest). Both photodetectors work on the same wavelength (corresponding to an absorption wavelength of the target gas analyte) resulting in a sensor that is relatively stable and produces reliable results.
The instrument configurations described above present some serious design drawbacks when attempting to modify them for portable, in-situ field use, beyond the laboratory walls. For example, requiring a separate, sealed gas reference cell containing an inert gas results in an instrument configuration that is expensive, bulky, heavy and unwieldy. Similarly, using an incandescent bulb as the source of radiation in a portable instrument does not make sense because incandescent bulbs, which provide the necessary wide wavelength radiation band, result in a radiation source that is slow to respond (typically, the response time is more than 100 milliseconds) and requires significant power (200 milliwatts or more). As such, the instrument configurations described above are not suitable for portable, low power sensors which ideally should operate at a power consumption of no more than 1-2 milliwatts.
To resolve these drawbacks, portable infra-red sensors have been developed where semiconductors are used as both radiation sources and radiation detectors. The radiation sources are semiconductors that behave as light emitting diodes (LEDs). Likewise, the radiation detectors are semiconductors that behave as photodetectors (PDs). A number of such infra-red LED sensors are disclosed in the Tkachuk U.S. Letters Pat. No. 7,796,265 titled Optical Absorption Gas Analyzer, (the “265” patent), which is hereby incorporated by reference in its entirety as if more fully set forth herein.
The '265 patent discloses an analyzer comprising a chamber for containing the sample in use; a radiation source assembly arranged to emit radiation into the chamber; a first radiation detector assembly arranged to detect radiation transmitted along a first optical path through the chamber; a second radiation detector assembly arranged to detect radiation transmitted along a second optical path through the chamber, wherein the length of the second optical path which the sample can intercept is shorter than that of the first optical path; and a processor adapted to generate a sensing signal SS based on the detected radiation transmitted along the first optical path and a reference signal SR based on the detected radiation transmitted along the second optical path, and to determine the concentration of the target gas in the sample based on a comparison of the sensing signal with the reference signal.
By arranging for radiation to be detected along a second optical path which is shorter than the first, the '265 patent provides a reference channel which operates using the same radiation as the signal channel, yet does not require the provision of a separate (inert) cell, since both optical paths pass through the same chamber. The relatively short length of the second optical path with which the sample can interact (compared to that of the first optical path) means that absorption in the reference channel is suppressed and can be used to accurately compensate for drift. Preferably, the length of the second optical path with which the sample can interact is made as short as possible, and in any case significantly shorter than that of the first optical path. As a result any losses caused by absorption in the reference path will be small.
The '265 patent further discloses that LEDs or any other fast-response radiation source (a response time of less than or equal to 100 milliseconds, preferably less than 1 milliseconds, still preferably less than 50 microseconds) can be used as the radiation source assembly arranged to emit radiation into the chamber, since both the signal and the reference channels can operate at the same or overlapping wavebands. Further, photodetectors and preferably photodiodes, can be used as the first and second detector assemblies.
Based on the foregoing, it appears that use of LEDs in portable infra-red instrument configurations may seem ideal. After all, LEDs are very fast; their response time is of the order of less than one micro-second. They provide greater selectivity of radiation. They have corrosive medium stability and longer periods of service and operation. And during all of that they consume very little power.3 3 Sotinikova, G. Y. et, al., Performance analysis of diode optopair gas sensors, Proc. Of SPIE, 2009, Vol. 7356 735617; Sotnikova, G. Y. et. al., Low Voltage CO2-Gas. Sensor Based on Mid-IR Immersion Lens Diode Optopairs: Where are we and how far can We go? IEEE Sensors Journal, 2009; Gibson D. et. al., A Novel Solid State Non-Dispersive Infrared CO2 Gas Sensor Compatible with Wireless and Portable Deployment. Sensors, 2013, 13, 7079-7103.
By way of background, LEDs are semiconductor emitters. Semiconductor emitters and detectors appear to be the most prospective candidates for a optical absorbance gas analyzers and sensors due to reliable processing technology of semiconductors, high repeatability and precision of the semiconductor devices. The technology of molecular beam epitaxy (MBE) allows atomic level precision and makes possible production of multi-layer semiconductor structures while preserving the crystalline order throughout the entire structure.
One of the most important characteristics of the semiconductor material is the magnitude of its so-called “band gap”. The band gap represents the energy range which is forbidden for the charge carriers inside the material. The band gap separates two allowed energy ranges: lower—“valence” band and higher—“conduction” band. In intrinsic semiconductors at low temperature the valence band is completely occupied by electrons while the conduction band is empty. To be excited to the conduction band, the electron has to obtain the minimum energy which is equal to the band gap energy or, simply, to the band gap. Excited electrons leave the vacancy in the valence band which behaves as a positively charged particle—“hole”. When the excited electron returns back to the valence band (or, in other words, recombines with the hole), the minimum energy it can return is the bandgap energy. If the energy is returned in the form of electromagnetic emission, the wavelength of the emission is determined by the band gap. So the band gap determines the emission spectrum of the emitter and the responsivity spectrum of the detector.
In semiconductor alloys, the band gap magnitude can be controlled through the alloy composition. Molecular Beam Epitaxy (MBE) allows more precise way of the effective band gap control through the dimensional quantization effect. In a very thin layer of semiconductor materials (with the thickness of decades of the atomic layers and less) the charge carrier energy is determined both by the material bandgap and the thickness of the layer. If this layer is able to retain the carriers it is called “quantum well”. The emitters based on quantum wells allow precise tuning of the band gap by variation of the quantum well width.
LEDs are some of the simplest semiconductor light emitters. Their composition and structure is such that when a current is applied to them, they emit light. As is shown in FIG. 1, a typical LED comprises two layers of semiconductor material. Each layer has a different type of conductivity, i.e., either n-type conductivity or p-type conductivity.
The conductivity type depends on the type of dopant introduced into the semiconductor material during its formation. Dopants are distinguished into “donors” and “acceptors.” Introduction into the semiconductor material of dopants, which are “donors”, produces an excess of mobile electrons, thereby resulting in a conductivity type which is negative: n-type. Introduction into the semiconductor material of dopants, which are “acceptors”, leads to a majority of mobile holes, thereby resulting in a conductivity type which is positive: p-type.
As is shown in FIG. 2, in the region near the boundary between n- and p-type materials the electrons recombine with the holes and a depletion region is formed. The concentration of mobile carriers in the depletion region is reduced. The depletion region is also characterized by built-in electric field. When the external bias is zero, the diffusion currents of holes into the n-type layer and electrons into the p-type layer are compensated by the drift currents produced by the built-in electric field, so the net current is zero. When positive bias (plus to p-type, minus to n-type) is applied to the LED, as is shown in FIG. 3, the drift component of the current is suppressed and the net diffusion current appears in the structure. Electrons and holes meet in the depletion region and recombine releasing energy in the form of photons. This effect is called electroluminescence
The place where most of the electrons and holes recombine is called the emitter region or active region of the LED. The color of the light (corresponding to the energy of the photons released) and the wavelength of the LED radiation, is determined by the energy band gap (the energy difference in electron volts between the top of the valence band (n-type) and the bottom of the conduction band (p-type) of the LED.
Applications of semiconductor hetero-structures have led to breakthroughs in the technology of light emitters. A semiconductor hetero-structure is a layered semiconductor crystal. The layers have different band gaps but same lattice constant, so crystalline order is preserved throughout the entire stack of layers. Introduction of narrow band gap layers (quantum wells), which can accumulate injected electrons and holes provide for a high recombination efficiency and precise control of the emission wavelength. A typical band diagram of the quantum well (QW) LED is presented in FIG. 5.
Molecular Beam Epitaxy (“MBE”) growth technology allows fabrication of quantum well LEDs, which can be used to create efficient cascaded LEDs. A cascaded LED structure includes several active regions connected in series. This injection scheme allows electron recycling so, one electron can produce more than one photon. Cascaded LEDs have increased power at a fixed current. Methods for the fabrication of cascaded LEDs are disclosed in the following articles, Jung, et. al., Light-Emitting Diodes Operating at 2 μm With 10 mW Optical Power, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 23, Dec. 1, 2013; Prineas, et. al., Cascaded active regions in 2.4 μm GalnAsSb light-emitting diodes for improved current efficiency. Applied Physics Letters 2006, 89, 211108; Crowder, J G., et. al., Mid-infrared gas detection using optically immersed, room-temperature, semiconductor devices. Meas. Sci. Technol. 2002, 13, 882-884; Li, W. et. al., InGaAsSbN: A dilute nitride compound for midinfrared optoelectronic devices. Journal of Applied Physics. 2003, Vol. 94, No. 7, 4248-4250; Ashley, T. et. al., Uncooled InSb/In1-xAlxSb mid-infrared emitter. Applied Physics Letters 1994, 64, 2433-2435; Shterenga, L. et. al., Type-I quantum well cascade diode lasers emitting near 3 m. Applied Physics Letters 2013, 103, 121108; Krier, A. et. al., The development of room temperature LEDs and lasers for the mid-infrared spectral range. Phys. Stat. Sol. (a), 2008, 205, No. 1, 129-143.
Likewise, by way of background, conventional photovoltaic detectors, i.e., photo-diodes (“PD”) are fabricated in a manner that is similar to the fabrication of LEDs. In contrast to LEDs, however, PDs absorb photons and produce an electrical signal whose magnitude is detennined by the intensity of the absorbed emission. So PDs operate in a manner that is the reverse of the operation of LEDs.
Conventional photo-diodes like LEDs comprise two layers of semiconductor material. Each layer has a different type of conductivity, i.e., either n-type conductivity or p-type conductivity. Like in LEDs, the conductivity type of each of the layers of the photo-diodes depends on the type of dopant introduced into the semiconductor material during its formation. Introduction into the semiconductor material of dopants, which are “donors”, produces an excess of mobile electrons, thereby resulting in a conductivity type which is negative: n-type. Introduction into the semiconductor material of dopants, which are “acceptors”, leads to a majority of mobile holes, thereby resulting in a conductivity type which is positive: p-type.
As a result, like in LEDs, the photo-diodes have a depletion region. Light in the form of photons absorbed in the depletion region of a photo-diode generates electrons and holes which are separated by the built in electric field. A schematic showing this operation principle of a photovoltaic detector is shown in FIG. 4. The motion of the generated electrons and holes in the depletion region creates a current whose signal can be measured precisely; but only when the signal to electron and hole generation noise ratio is high.
In accordance with Beer's-Lambert Absorption law (see discussion above), when the radiation produced by an LED passes through an analyte gas sample, the analyte gas sample will absorb energy as the radiation comes in contact with it. The amount of energy the analyte gas sample absorbs will vary as a function of frequency and wavelength of the radiation. The closer the frequency and wavelength of the radiation is to the corresponding frequency and wavelength of absorption line of the analyte gas sample, the larger the absorption of energy by the analyte gas sample. The larger the absorption the better and more reliable the signal to the photodiode detector, by which the detection and measurement the analyte gas sample is achieved.
Thus, in accordance with the foregoing, the following must be present for optimal optical gas sensor performance. First, the LED used as a radiation source and the photodetector used to read the absorption signal must be optically and spectrally matched (optopair). And, second, the energy and wavelengths of the radiation generated by the LED and directed on the analyte gas sample must coincide, correspond to and match the energy and wavelength of the maximum absorption line within the analyte gas sample's absorption spectrum.
However, due to the manner of LED production, it is very difficult to manufacture LEDs that consistently and reproducibly generate radiation whose energy and wavelengths coincide, correspond to and match the maximum absorption line within the gas' absorption spectrum. That is the reason why, many times optical gas sensors of the type described herein above are equipped with bandpass filters. The filters cut off all of the radiation emissions outside the target sample gas band, and enhance the relative magnitude of the absorbed emission. The filter is characterized by the spectral transmission function F(ω))=Θ(ω−ω1)−Θ(ω−ω2) where Θ is the step function.
But the use of LEDs and the photodetectors in portable infra-red instrument configurations, such as the optical gas sensors discussed above, is not without problems even if there are bandpass filters. First, the photodetectors discussed herein above produce noise during the generation of electrons and holes that could mask and interfere with the electric current generated thereby.
Second it is very well known that both the LEDs and the photodetectors are inherently sensitive to temperature changes. As the temperature changes, the LED emission spectrum broadens, its maximum spectral position shifts, and its intensity drops. Likewise, the amplitude and spectral position of the photo-detector responsivity drops and shifts respectively. Thus, these gas sensors cannot be reliable and suitable for the detection of gases in multiple environments having different temperatures. “The temperature shift of spectral characteristics of a source and detector of radiation inherent to all semiconductor elements and photoresistors without exception leads to changes of the optical sensor output signal and consequently, to errors in calculating the gas concentration.”4 4 Sotnikova, G. Y. et. al., Low. Voltage CO2-Gas Sensor Based on III-V Mid-IR Immersion Lens Diode Optopairs: Where are we and how far can We go? IEEE Sensors Journal, 2009
More specifically, when an optical gas sensor equipped with an LED as its radiation source is taken out of an environment having one temperature and placed in an environment having a different temperature, the spectral position and intensity of the LED's radiation emission changes. As the temperature increases, the LED's band gap energy changes, resulting in a decrease of its luminescence intensity and a shift in its spectral position. And when it does shift, it will no longer correspond to the absorption line of the analyte gas sample, resulting in a much smaller absorption of radiation by the gas sample, a poor signal to the photodiode detector, and an unreliable detection and measurement of the gas.
In other words, when the temperature increases the spectrum maximum of the LED's emission radiation targeting the gas sample analyte will no longer coincide or correspond to the maximum of the absorption spectrum of the gas sample. As a result, the gas sample will absorb less of the radiation that comes in contact with it. The less the absorption of radiation by the gas sample, the less the photo-detector signal will be. The less the photo-detector signal, the more unreliable the detection and measurement of the gas sample will be.
Likewise, as the temperature increases, the peak of the photo-detector responsivity curve will shift to a lower wavelength side and its amplitude will decrease. As a result, the LED and the Photodetector will no longer be spectrally matched. This in turn will produce a signal that either does not register or is so small in value that it once again, it will be unreliable for the detection and measurement of the target sample gas.
Thus, as can be seen from the foregoing, an LED's power output and emitted radiation spectrum depend greatly on and will vary with the temperature of the environment the LED is being used in. Further, the LEDs' sensitivity to temperature changes is such an inherent characteristic of the LEDs' composition and structure, that many of the attempts to design around it up until now have used a totally different approach. See for example U.S. Letters Pat. No. 8,649,012 issued to Beckman et. al. and titled Optical Gas Sensor which attempts to solve the foregoing problem but in a manner that is totally different from the present invention. It discloses a sensor having a light-emitting diode, a photo-sensor, a measuring section between the light-emitting diode and the photo-sensor, and a control and analyzing unit, which is set up to determine the concentration of a gas in the measuring section from the light intensity measurement by the photo-sensor. The control and analyzing unit is set up to measure the forward diode voltage over the light-emitting diode at a constant current, to determine the temperature of the light-emitting diode from the detected forward diode voltage over the light-emitting diode by means of a preset temperature dependence of the forward diode voltage, and to apply a correction function as a function of the light-emitting diode temperature determined, with which the measurement is converted to that of a preset temperature of the light-emitting diode. See also U.S. Application Pub. No. US 2007/0035737 A1.
On the basis of the foregoing, it becomes evident that the LED radiation source in a portable infra-red gas sensor together with a photo-diode photodetector, chosen for optimal power output and emitted radiation wavelength at a certain temperature, will not have the same power output and emitted radiation at a different temperature. Thus, a single sensor will not be reliable and suitable for use in a wide environment temperature range. In other words, one who wants to use an infrared LED gas sensor in a room that is at 25° Celsius, would not be able to reliably use the same gas sensor in an environment that is at 60° Celsius. Use of a single infra-red LED gas sensor in multiple environments with different temperatures is unreliable.
A way to resolve the LED's reliability issue due to its sensitivity to temperature fluctuations, different from U.S. Letters Pat. No. 8,649,012 issued to Beckman (see discussion herein above) would be to use multiple sensors for measurements of gases at different temperatures. But that would result in having to keep multiple sensors at hand, one for each temperature at which a gas would have to be identified and quantified; something that would be extremely cumbersome, expensive and highly impractical.
Resolving the drawbacks in the prior art as discussed herein above is of paramount importance because among other things, reliable techniques for detecting methane and monitoring its concentration are in high demand in many industrial areas such as mining, construction, transportation of carbohydrate fuels and many others. The techniques need to be accurate enough to detect methane concentration below the methane lower explosive limit (LEL, 5% in atmosphere). A minimum accuracy of 1% is a must.
Methane has a low chemical activity. Accordingly, the chemical methods of methane detection are not effective and absorption spectroscopy through optical detection as described herein above, appears to be a promising path for methane sensing and analysis. However, any methane sensor implementing absorption spectroscopy must provide low power consumption and portability, with a 5-10 cm limit for the optical path of the probe emission. Further, it has to be able to feel changes of as low as 0.1% in the probe emission intensity. Finally it has to be based on detection of the change in the intensity of the probe emission in the absorption spectral range which is typical for methane only. Since no other atmospheric gases absorb in that range, any change of light intensity within the methane spectral range will indicate presence of methane and in accordance with Beers-Lambert Law of Absorbance, the magnitude of the absorption will be proportional to the concentration of methane gas sample analyte. Thus, the emission spectrum of the optopair light source should match to the optopair detector responsivity spectrum and the methane absorption band.
One of the strongest and most prominent vibration-rotation absorption bands of methane is near wavelength λ=3.3 μm (˜3000 cm−1), which is within the mid-IR spectral range. So the emitter and detector should have their spectral characteristics peaked near 3.3 μm.
The bandgap of the emitter and detector material for methane sensing has to be around 0.367 eV which corresponds to λ=3.3 μm. This band gap belongs to relatively narrow band materials and it means strong temperature dependence of the materials' properties in the temperature range 240-330K. (−40 C to +60 C). This is a serious obstacle for developing a semiconductor based optopair which can reliably operate in this temperature range, which is essential for outdoor stand-alone methane sensors. Upon information and belief, no semiconductor methane sensors operating in this temperature range exist.